The search for new donor: acceptor pairs for organic photovoltaic (OPV) applications is highly important for socio-economic and environmental reasons.
From the point of view of performance and stability, the composite couple of poly(3-hexylthiophene):[6,6]-phenyl-C61-methyl butyrate (P3HT:PCBM) has long been the standard bearer.
However the poor electronic correlation between the donor (P3HT) and the acceptor (PCBM), combined with numerous morphological instabilities at the heart of the heterojunction are two of the major problems that are tackled through diverse solutions proposed in the literature.
The search for a better correlation in the electronic properties ranges from minor modifications in the chemical structures of one of the pair components to complete replacement of the donor or acceptor molecule.
Due to the inherent difficulties of synthesising or grafting groups onto fullerene, the precursor of PCBM, numerous authors have sought to replace it.
Accordingly, graphene-based materials have rapidly appeared as good acceptor candidates, primarily because of their remarkable semiconducting properties.
In particular, Jones et al. in “Synthesis of electron-poor-hexa-peri-hexabenzocoronenes”, Chem. Commun, 2012, 48, 8066-8068, disclose a series of hexabenzocoronene (HBC) derivatives containing Br, F, CF3 and aryl substituents said to be appropriate acceptors to be substituted to PCBM in the composite couple of P3HT:PCBM.
However, the short current circuit Jsc of this composite couple is quasi-null so that the efficacy of the OPV devices using this composite couple is also quasi-null. The efficacy depends from the product (Voc×Jsc×FF) divided by the power in, where FF is the fill factor and the Voc is the open circuit Voltage. The FF is the product of the maxima of the voltage and current divided by the product of the Voc and the Jsc. Furthermore the donor and acceptor are a blend and are not covalently connected, therefore the charge transfer is hindered by this nanoscale disorganisation.
Also, L. Schmidt-Mende et al. in, Science 111, 293, 2001, describe columnar systems in which the donor is an HBC modified by alkyl chains and the acceptor is a alkyl modified perylene and in which all other substituents are H, such an HBC. However, the electronic levels of the HBC moieties have not been modified through careful change of substituent groups to optimize the electronic efficacy nor have the donor and acceptor been covalently linked to provide structural stability and self-organization to optimize charge-collation and transfer to electrodes.
But, materials in organic photovoltaic cells should ideally be adapted and optimized for maximum efficiency at each stage of the process of converting solar energy into electricity.